Portable Liquid Crystal Displays (LCDs) that are used for laptop computers and other personal computing and communication devices require a backlighting unit that is compact and relatively efficient. In many single-viewer applications of LCD displays, only a narrow viewing angle is required. By providing illumination directed primarily toward the viewer, that is, in a normal direction, an efficient backlighting unit meets this need and requires less energy, thus conserving battery power. In addition, narrow viewing angle LCDs safeguard privacy, which can be critical when the LCDs are used in public.
One solution for providing illumination of the required type uses light from a lamp that is directed into a Light Guid Plate (LGP), typically in combination with one or more types of films for redirecting and conditioning the light. Different types of LGP have been developed for this purpose. Some types of LGPs use printed dots or other surface-scattering mechanisms to provide scattered, diffused light over a broad range of angles. Other types of LGPs are tapered or wedge-shaped and use Total Internal Reflection (TIR) to provide output light that is generally collimated, with a dominant ray or principal ray that is at a relatively large angle relative to normal. Taking advantage of both approaches, the CLAREX® HSOT (Highly Scattering Optical Transmission) light guide panel available from Astra Products, Baldwin, N.Y., uses a type of LGP that employs some forward-scattering to provide generally uniform backlighting, and is tapered at the same time to provide illumination that is angularly biased away from normal. To redirect this light toward the viewer, the HSOT light guide panel employs a directional turning film.
In illumination apparatus that uses a conventional turning film, surface or volume scatterers are typically used in combination with a wedge angle in order to extract light that is already somewhat collimated in the direction parallel to the tubular Cold-Cathode Fluorescent Light (CCFL) source. Using the mechanism of frustrated Total Internal Reflection (TIR), the wedge-shaped light guide provides light, at a glancing angle relative to the light guide surface, to a turning film. HSOT light guide panels and similar types of directional turning films use arrays of prismatic structures, arranged in various combinations, to redirect light exiting from a light guiding plate toward normal, that is, the 0-degree direction, relative to the two-dimensional surface.
Referring to FIG. 1, the overall function of a light guiding plate 10 in a display apparatus 30 is shown. Light from a light source 12 is incident at an input surface 18 and passes into tapered light guiding plate 10. The light propagates within light guiding plate 10 until Total Internal Reflection (TIR) conditions are frustrated and then, possibly reflected from a reflective surface 42, exits light guiding plate 10 at an output surface 16. Relative to normal N, the exit angle of light from light guiding plate 10 is fairly large, typically in the range from about 40 to 88 degrees. This light then goes to a turning film 22 and is redirected toward normal to illuminate a light gating device 20 such as an LCD or other two-dimensional backlit component.
For distributing the light along a two-dimensional surface, light guiding plate 10 and its support components are typically designed to provide both redirection of the light and some amount of collimation that reduces divergence of the beam angle. For example, U.S. Pat. No. 5,854,872 entitled “Divergent Angle Rotator System and Method for Collimating Light Beams” to Tai discloses a light guiding plate that uses an array of elongated microprisms to redirect and collimate light from one or more light sources. In the device disclosed in the Tai '872 patent, the light guiding plate has a first set of prismatic structures on the light output side elongated in one direction to provide collimation and a second set of prismatic structures on the opposing side elongated in the orthogonal direction and providing collimation and TIR reflection. There are a number of variations applied to this basic arrangement. For example, U.S. Pat. No. 6,576,887 entitled “Light Guide for use with Backlit Display” to Whitney et al. discloses a light guide optimized for uniformity, in which structures on the output surface of a turning film 22 may be randomly distributed to provide a more uniform output. U.S. Pat. No. 6,707,611 entitled “Optical Film with Variable Angle Prisms” to Gardiner et al. discloses adaptation of an optical turning film with an arrangement that reduces perceived ripple.
Turning films are described, for example, in U.S. Pat. No. 6,222,689 entitled “Surface Light Source Device and Asymmetrical Prism Sheet” to Higuchi et al.; in U.S. Pat. No. 5,126,882 entitled “Plane Light Source Unit” to Oe et al.; and in U.S. Pat. No. 6,746,130 entitled “Light Control Sheet, Surface Light Source Device and Liquid Crystal Display” to Ohkawa. A number of approaches for optimizing the design of directional 2-D turning films are described in patent literature. For example, the '611 Gardiner et al. disclosure describes optimized geometric arrangements for the prism surface on the incident light surface of a turning film. U.S. Pat. No. 6,669,350 entitled “Planar Light Source System and Light Deflecting Device Therefor” to Yamashita et al. discloses an arcuate distribution of elongated prismatic structures on the incident light surface. U.S. Pat. No. 5,600,462 entitled “Optical Film and Liquid Crystal Display Device Using the Film” to Suzuki et al. discloses a conventional arrangement in which a turning film has elongated prismatic structures on the incident light surface and diffusing elements on the emitting light surface. For improving luminous intensity, however, this type of arrangement that employs both diffusing and light-directing elements in the same optical film is necessarily somewhat a compromise.
The conventional turning film redirects the incident light from light guiding plate 10 toward normal, over a small range of angles. Light outside this range is redirected at near-normal angles. One optimization strategy that has been used for expanding the range of angles of redirected light relates to prism geometry on the input side of turning film 22. FIG. 2A shows a small portion of turning film 22 in which prismatic structure 24 has a substantially isosceles shape in cross section. That is, on the input side of turning film 22, peak half-angle α equals peak half-angle β. For this and subsequent figures, the angle of a ray C for light emitted from turning film 22 is relative to normal N, as shown.
The graph of FIG. 2B shows the luminous intensity response relative to Normal (0 degrees) for a turning film 22 with this arrangement. An incident light luminous intensity curve 45 plots luminous intensity vs. angle for light that is output from the LGP and incident to turning film 22. As this curve shows, the light that is incident to turning film 22 is highly directional and has peak intensity at approximately 70 degrees from Normal (0 degrees). An output light luminous intensity curve 46 then shows the effect obtained at the output of turning film 22. Here, the intensity is substantially the same as the output from the LGP, but the angle is shifted, now centered around 0 degrees. This favorable shift of the angle toward normal helps to maximize the overall efficiency of the backlighting illumination system.
U.S. Pat. No. 6,222,689 entitled “Surface Light Source Device and Asymmetrical Prism Sheet” to Higuchi et al. discloses a turning film in which the relative sizes of peak half-angle α and peak half-angle β are unequal. A film of this type can provide improved performance when properly matched to the angle of incident light. FIG. 3A shows, in cross-sectional view, a small section of turning film 22 in which prismatic structure 24 has peak half-angle α less than peak half-angle β. FIG. 3B shows a typical luminous intensity curve 48 for light to each side of a 0 degree normal viewing angle using the altered geometry of FIG. 3A. As shown, luminous intensity increases on the order of greater than about 10% have been obtained using this angular adjustment. Other attempts to improve turning film performance by modifying the shape of prismatic structures, such as in U.S. Pat. No. 6,669,350 entitled “Planar Light Source System and Light Deflecting Device Therefor” to Yamashita, et al. for example, have provided moderate levels of improvement in luminous intensity.
The fabrication of double-sided optical films, including some types of turning films and various brightness enhancement articles, has been addressed in a number of ways. A number of approaches apply one or more coating materials to a moving web that acts as a carrier, using patterned rollers for forming the needed surface features. For example, U.S. Pat. No. 6,628,460 “Lens Sheet and Method for Producing the Same” to Ookawa et al. discloses a double-sided lenticular screen in which rows of lens structures on opposite incident and emitting surfaces of the screen extend in substantially parallel directions. For each side of the film, a curable resin is applied to a transparent substrate carrier, then shaped and cured to form light-redirecting elements. Other approaches that apply a curable polymer onto a transparent carrier are shown, for example, in U.S. Patent Application Publication No. 2006/0210770 entitled “Microreplicated Article with Defect-Reducing Surface” by Nelson et al.; in International Publication WO 2005/025837 entitled “Apparatus and Method for Producing Two-Sided Patterned Webs in Registration” by Huizinga et al.; and in U.S. Patent Application Publication No. 2006/0209428 entitled “Microreplicated Article with Moire Reducing Surface” by Dobbs et al.
Still other methods that have been used for fabrication of double-sided optical films include lamination. Separate sheets are formed having the features needed for opposite sides of the film. These sheets are then laminated onto a carrier or directly to each other to form the finished article. Embossing techniques can also be used to form a double-sided optical film onto a moving web.
Each of these conventional approaches, however, presents some problems, particularly where it is necessary to obtain precise registration of structures formed on opposite sides of the optical film. For applications using patterned rollers, such as those of the '460 Ookawa et al. or '9428 Dobbs et al. disclosures, close radial synchronization must be maintained between pairs of patterned rollers in order to make sure that features formed on each surface are in precise register. Lamination solutions can prove to be challenging, since the heat generated during lamination transfer can impact dimensional accuracy for the plastic sheet materials that are used.
With increased demands for more compact packaging of electronic display apparatus and for improved brightness efficiency, and with little promise of dramatic improvement to existing light-scattering approaches for backlight delivery, there is a compelling need for light redirection solutions that provide a high degree of collimation in order to significantly increase brightness in the display viewing direction. For implementation of these solutions, improved fabrication techniques are also needed. In particular, it is desired to have a backlight with a turning film that can provide light from a sidelight that provides a peak output angle of ±10° from normal to the LC cell and an optical gain of at least 1.25.